1. Field of the Invention
The present invention relates to a cryogenic refrigerator and, more particularly, a refrigerator of the refrigerant-accumulating type.
2. Description of the Related Art
Various kinds of cryogenic refrigerators are now on the market. One of them is of the Gifford-McMahon type. This refrigerator is usually arranged as shown in FIG. 1.
The refrigerator comprises generally a cold head 1 and a coolant gas introducing and discharging system 2. The cold head 1 includes a closed cylinder 11, a displacer 12 freely reciprocating in the cylinder 11, and a motor 13 for driving the displacer 11.
The cylinder 11 includes a first large-diameter cylinder 14 and a second small-diameter cylinder 15 coaxially connected to the first cylinder 14. The border wall between the first 14 and the second cylinder 15 forms a first stage 16 as a cooling face and the front wall of the cylinder 15 forms a second stage 17 which is lower in temperature than the first stage 16. The displacer 12 includes a first displacer 18 reciprocating in the first cylinder 14 and a second displacer 19 reciprocating in the second cylinder 15. The first and second displacers 18 and 19 are connected to each other in the axial direction of the cylinder 11 by a connector 20. A fluid passage 21 is formed in the first displacer 18, extending in the axial direction of the displacer 18, and a cooling member 22 formed of copper meshes or the like is housed in the fluid passage 21. Similarly, a fluid passage 23 is formed in the second displacer 19, extending in the axial direction of the displacer 19, and a cooling member 24 formed of lead balls or the like is formed in the fluid passage 23. Seal systems 25 and 26 are located between the outer circumference of the first displacer 18 and the inner circumference of the first cylinder 14 and between the outer circumference of the second displacer 19 and the inner circumference of the second cylinder 15, respectively.
The top of the first displacer 18 is connected to the rotating shaft of the motor 13 through a connector rod 31 and a Scotch yoke or crankshaft 32. When the shaft of the motor 13 is rotated, therefore, the displacer 12 reciprocates, as shown by an arrow 33 in FIG. 1, synchronized with the rotating shaft of the motor 13.
An inlet 34 and an outlet 35 for introducing and discharging coolant gas extend from the upper portion of one side of the first cylinder 14 and are connected to the coolant gas introducing and discharging system 2. The coolant gas introducing and discharging system 2 serves as a helium gas circulating system, comprising connecting the outlet 35 to the inlet 34 through a low-pressure valve 36, a compressor 37 and a high-pressure valve 38. Namely, this system 2 is intended to compress low-pressure (about 5 atm) helium to high-pressure (about 18 atm) helium by the compressor 37 and send it into the cylinder 11. The low- and high-pressure valves 36 and 38 are opened and closed, as will be described later, in a relation to the reciprocation of the displacer 12.
The portions in the refrigerator where cooling is effected or which act as cooling faces are the first and second stages 16 and 17, which are cooled or refrigerated to about 30 K. and 10 K., respectively, when no thermal load is present. Therefore, a temperature gradient ranging from a normal temperature (300 K.) to 30 K. exists between the top and bottom of the first displacer 18 and a temperature gradient ranging from 30 K. to 10 K. exists between the top and bottom of the second displacer 19. These temperature gradients, however, are changed by thermal loads at the step stages and it usually ranges from 30 K. to 80 K. at the first stage 16 while it ranges from 10 K. to 20 K. at the second stage 17.
When the motor 13 starts its rotation, the displacer 12 reciprocates between top and bottom dead centers. When the displacer 12 is at the bottom dead center, the high-pressure valve 38 is opened, allowing high-pressure helium gas to flow into the cold head 1. The displacer 12 then moves to the top dead center. As described above, the seal systems 25 and 26 are arranged between the outer circumference of the first displacer 18 and the inner circumference of the first cylinder 14 and between the outer circumference of the second displacer 19 and the inner circumference of the second cylinder 15, respectively. When the displacer 12 moves to the top dead center, therefore, high-pressure helium gas flows into a first stage expansion chamber 39 formed between the first 18 and the second displacer 19 and then into a second stage expansion chamber 40 formed between the second displacer 19 and the front wall of the second cylinder, passing through the fluid passage 21 in the first displacer 18 and the fluid passage 23 in the second displacer 19. While flowing in this manner, high-pressure helium gas is cooled or refrigerated by the cooling members 22 and 24, so that high-pressure helium gas flowing into the first stage expansion chamber 39 can be cooled to about 30 K. and high-pressure helium gas flowing into the second stage expansion chamber 40 can be cooled to about 8 K. Here, the high-pressure valve 38 is closed and the low-pressure valve 36 is opened. When the low-pressure valve 36 is opened, high-pressure helium gas in the first stage expansion chamber 39 and the second stage expansion chamber 40 is expanded and cooling is effected. The first stage 16 and the second stage 17 are cooled by this cooling phenomenon. Then, the displacer 12 moves to the bottom dead center again and helium gas in the first stage expansion chamber 39 and the second stage expansion chamber 40 is removed as the movement of the displacer 12. The expanded helium gas is warmed by the cooling members 22 and 24 while passing through the fluid passages 21 and 23, and is at an ordinary temperature and discharged. Thereafter, the above-mentioned cycle is repeated and the refrigerating operation is performed. This type of the refrigerator is used for cooling a superconducting magnet or an infrared sensor, or as a cooling source of a cryopump.
However, the above-structured conventional cryogenic refrigerators have the following problems. Specifically, the cylindrical fluid passage 23 is formed in the second displacer 19 and the inside of the passage is filled with the ball or grain-like cooling member 24. Speed distribution in helium gas flowing through the passages which were filled with balls or grains was measured and it was found that velocity of flow was the lowest in the center of the flow of helium gas and that it became higher and higher moving away from the center of the flow of helium gas outward in the radial direction thereof. This means that a larger amount of helium gas flows only into some area of the cooling member 24 and that the cooling member 24 must exchange heat with excessive helium gas at this area thereof when heat exchange is to be done between helium gas and the cooling member 24. This teaches us that the cooling member 24 is not efficiently used. Therefore, cooling efficiency (or heat exchanging efficiency achieved by a cooling means) is reduced at the area of the cooling member, thereby resulting in reducing refrigerating capacity at a certain temperature.
The conventional refrigerators arranged as shown in FIG. 1 have a problem as described below. The seal system 25 prevents helium gas from flowing from the normal temperature section to the first expansion chamber 39 and vice versa, passing through a clearance between the first cylinder 14 and the first displacer 18, while the seal system 26 prevents helium gas from flowing from the first stage expansion chamber 39 to the second stage expansion chamber 40 and vice versa, passing through a clearance between the second cylinder 15 and the second displacer 19. These seal systems 25 and 26 are used in helium gas of high purity (99.99%) and a lubricating material such as grease cannot be used in them because it contaminates helium gas. Particularly the seal system 26 is located at the low temperature section (30 to 80 K.) and has a shape like the piston seal. Providing that the first stage expansion chamber 39 has a temperature of 30 K. while the second stage expansion chamber 40 has a temperature of 10 K. and that helium gas leaks at some portion of the seal system 26, helium gas of 30 K. will enter into the second stage expansion chamber 40 without contacting the cooling member 24 in the second displacer 19 and helium gas of 10 K. will enter into the first stage expansion chamber 39. As the result, the temperature of the first stage 16 falls and that of the second stage 17 rises. FIG. 3 shows, as results calculated, the relation between the ratio of the amount of helium gas leaked through the seal system 26 (or ratio of the amount of helium gas flowing into the second stage expansion chamber 40 through the seal system 26 relative to the total amount of helium gas flowing into the chamber 40 through the passage) and the temperature of each of the first and second stages 16, 17. As apparent from FIG. 3, helium gas leaked at some portion of the seal system 26 adds large influence to the temperature of each of the stages 16 and 17. Same thing can also be said about the seal system 25.
In the conventional refrigerators, the seal system 26 used comprises fitting a turn of sealing 28 provided with overlapped ends 30 as shown in FIG. 6 into a ring-shaped groove 27 on the outer circumference of the second displacer 19 and arranging a spring ring 29 on the backside of the sealing 28 to urge the sealing 28 against the second cylinder 15, as shown in FIGS. 4 through 6. In the case of the seal system 26 having the above-described arrangement, a considerable amount of helium gas is allowed to leak through the overlapped ends 30 of the sealing 28, thereby causing the temperature of the second stage 17 to rise. This results in reducing refrigerating capacity at a certain temperature.
Providing that the temperature of the first stage expansion chamber 39 is 30 K. while that of the second stage expansion chamber 40 is 10 K. and that helium gas leaks through the sealing portion, helium gas of 30 K. will enter into the second stage expansion chamber 40 while helium gas of 10 K. into the first stage expansion chamber 39, without fully contacting the cooling member 24 in the second displacer 19. As a result, the temperature of the first stage 16 lowers while that of the second stage 17 rises. FIG. 7 shows, as results calculated, what relation exists between the ratio of the amount of helium gas leaking through the clearances (or ratio of the amount of helium gas flowing into the second stage expansion chamber 40 through the sealing portion relative to the total amount of helium gas flowing into the chamber 40 through the cooling member) and the temperature of each of the first and second stages 16 and 17. As apparent from FIG. 7, helium gas leaking through the sealing portion between the displacer and the cooling member adds large influence to the temperature of each of the stages.
The conventional refrigerators arranged as shown in FIG. 1 have another problem as described below. When magnetic material is used as a part or whole of the cooling member 24 in the second displacer 19, it is quite difficult to process the magnetic material into balls or meshes such as the cooling member 22 in the first displacer 18. The magnetic material is therefore melted to a bulky mass, which is ground and screened to grains each having a size of about 100 to 500 .mu.m. These grains substantially same in size are used as the cooling member. However, each of these grains has sharp edges and tips which are several .mu.m in size, and these sharp edges and tips are broken off the grains while the refrigerator is under operation. The cooling member 24 is covered by sheets of net at the top and bottom thereof not to drop from the second displacer 19, but these sheets of net have meshes each having a size of several tens .mu.m and fine edges and tips broken off the grains of magnetic material pass through these meshes of the nets together with helium gas. When the meshes of the nets which cover the top and bottom of the cooling member 24 are made smaller in size, however, the pressure loss of helium gas is increased. This is not a merit. The fine edges and tips of magnetic material dropped from the second displacer 19 adhere to the seal 25 to thereby increase the amount of helium gas which leaks through the seal 25. This lowers the refrigerating capacity of the refrigerator to a great extent. In addition, the fine edges and tips of magnetic material dropped come to the compressor 37, passing through the first displacer 18 and the valve 36. As the result, the valve 36 can be blocked and the compressor 37 can be damaged by them. When ground grains of magnetic material are used as the cooling member as described above, the capacity of the refrigerator is lowered and the refrigerator itself is damaged.
The conventional refrigerators arranged as shown in FIG. 1 have a further problem as described below. When the first and second displacers 18 and 19 are filled with the cooling members 22 and 24, clearances are caused between the cooling members and the displacers. When gas flows passing through these clearances, effective heat exchange cannot be carried out between the gas and the cooling member.